This is a cellphone picture I took back in mid-December of Belgrade’s fortress, Kalemegdan. The fortress stands on a ridge above where two rivers, the Danube and the Sava, meet. That’s a view to the south, from along a fortification wall. It’s one of my favorite places to walk around in and explore. Every cobblestone drips with history, starting with Romans in the second century and with many a battle, siege, and transformation since then, layer upon layer.

It got me thinking about a subj...